In order for a user of a device, such as a mobile telephone, to gain access to a service provided by a network operator, the user needs three things:                a billing relationship with a network operator whereby services are paid for in advance of them being consumed or where bills are issued and settled after the service has been consumed,        a device that is compliant to the type of technology the network operator intends to deploy and that operates at the same frequency as the operator, and        a Subscriber Identity Module (SIM) card that is activated by the network operator once the billing relationship is established.        
A SIM card securely stores a service subscriber key that is used to identify a user and allows a user to change device by simply removing the SIM card from one device and inserting it into another. A SIM card usually contains a unique serial number, an internationally unique number of the mobile user (IMSI), security authentication and ciphering information, temporary information related to the local network, a list of the services a user has access to and one or more passwords such as a personal identification number (PIN) and a PIN-unlock key (PUK).
Once a SIM card is loaded into a powered device, it will search for the nearest base station with the strongest signal in the operator's frequency band. The device then identifies itself/its user to the network and once this is successfully completed, the device is attached to the network operator's communication network.